singleton.php

<?php
abstract class CacheDriver {
    private static $instance = NULL;

    public static function getInstance()
    {
        $classname = get_called_class();

        if (self::$instance != NULL) return self::$instance;
        self::$instance = new $classname();
        return self::$instance;
    }
}

class MemcachedDriver extends CacheDriver {
}

$cacheDriver = MemcachedDriver::getInstance();